Liquid Crystal display projector apparatus are growing in popularity for use with computerized presentations, television and other home entertainment purposes, and the like. Therefore, many types of projection apparatus using light valves have been proposed; among these apparatus, a liquid crystal projection apparatus has recently been commercialized that displays images on a large screen by enlarging and projecting images displayed by miniaturized liquid crystal display (LCD) panels.
In projection displays, an image from an active device, such as an LCD, is projected onto a screen at a desired magnification. For color images, complementary images in three primary colors may be obtained from three light valves. The images from the light values are simultaneously projected and superimposed onto a screen, or displayed as a virtual image for direct viewing by the user. The optics used to generate color images in this manner typically comprise a dichroic beam splitter such as a color separation/combining prism set, and a polarizing beam splitter such as a polarizing beam splitting cube. The optical performance of the dichroic beam splitter must be very high quality for high resolution imaging. The crossed dichroic cube variety of beam splitter typically consists of four subcomponents assembled together in one cubic unit. Two of the three optical paths in this cube involve imaging the light valves through a split mirror without introducing aberrations. To produce an acceptable image, the optical and mechanical tolerances required for assembly of the four subcomponents must be held very precisely.
The color prism variety of beam splitter typically consists of three prisms assembled into a single compact unit. As with the dichroic cube, two of the three optical paths in the color prism involve imaging the light valve through a dichroic split mirror without introducing aberrations. The optical and mechanical tolerances for assembly of the three components (including in some cases a precisely controlled air gap) must similarly be very tightly controlled.
In any of the foregoing displays in which complementary images are superimposed on one another, a significant limitation on the contrast ratio of the display is the inability to precisely align both the optical images and the polarization axes of the multiple LCD panels. In order to ensure adequate fidelity of the projected color image, the image produced by each of the three light valves must be precisely aligned with the image produced by the other two of the light valves. Accordingly, in addition to maintaining tight tolerances on the dichroic beam splitter assembly, the translational and rotational position of the light valves themselves with respect to the dichroic beam splitter must be tightly controlled. In many cases, optical feedback from the light valves themselves are used during final assembly to ensure proper alignment. Liquid Crystal Display light valves, however, also experience manufacturing tolerances that cause the polarization state (i.e., the dominant polarization axis and the ellipticity) of light exiting the LCD to vary relative to its image. Although the aforementioned optical feedback alignment of the LCD images compensates for deviations from the ideal optical alignment, the process does not compensate for deviations from the ideal polarization state. The imperfect polarization states among the multiple LCD's in such a display permits substantial light leakage of pixels selected to be in their dark state, which leads to an overall reduction in the contrast ratio achievable by such a display and introduces undesirable color shifts. Rotating the LCD light valves to compensate for the polarization state of the light exiting the LCDs would improve contrast ratio, but would also produce an unacceptable misalignment of the LCD images.
What is needed then is a method for compensating for the polarization state of an LCD that is independent from the mechanical/optical alignment of the LCD.